1. Field of the Invention
The present invention relates generally to an actively controlled suspension system which controls suspension characteristics or stiffness of suspension system of an automotive vehicle for regulating vehicular height, an absorbing road shock and suppressing vehicular attitude change. More specifically, the invention relates to a fluid circuit for the actively controlled suspension system which assures safety even when a control system controlling the suspension system fails.
2. Description of the Background Art
U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987, the co-pending U.S. patent applications Ser. Nos. 052,934, 059,888, 061,368, 060,856 and 060,909 respectively filed on May 22, 1987, June 9, 1987, June 15, 1987, June 12, 1987 and June 12, 1987, and commonly assigned to the assignee of the present invention, propose actively controlled suspension systems which generally achieve vehicular height regulation for maintaining vehicular height level at a predetermined height range and, as well, achieve attitude change suppression for better driving stability and riding comfort. Such actively controlled suspension systems employs proportioning valve assemblies in a pressure circuit for adjusting fluid pressure in a working chambers defined within working cylinders disposed between vehicular bodies and suspension members rotatably supporting vehicular wheels.
Generally, the proportioning valve assembly employed in the foregoing U.S. Patent, and the co-pending U.S. patent applications, is disposed within a hydraulic circuit connecting a pressurized working fluid source unit and the working chamber. The proportioning valve assembly comprises a three way spool valve or poppet valve which is supplied substantially constant line pressure from the pressurized working fluid source and output adjusting pressure controlled by an electric actuator, such as an electrically driven proportioning solenoid and so forth. The actuator operates to adjust a pilot pressure to be exerted on the pressure controlling valve, such as the spool valve or poppet valve. A pilot chamber in which the pilot pressure is generated, is selectively connected to a control port connected to the working chamber of the working cylinder to receive therefrom the control pressure to be supplied to the working chamber as feedback pressure, and to a drain port for draining pressure to the pressurized working fluid source via a drain line.
The proportioning valve is associated with an electric control circuit including a microprocessor for processing control parameter indicative sensor signals for deriving a control signal value. The control circuit also includes a driver circuit receiving the control signal and producing a driver current which is variable of current value for adjusting the valve position and whereby adjusting the fluid pressure in the working chamber of the working cylinder.
As will be appreciated, the proportioning valve assemblies are provided for respective working cylinders disposed in front-left, front-right, rear-left and rear-right suspension systems which are mutually distanced in substantial length. This results in requirement of relatively long wirings for distributing control signals or driver current. Such wirings constantly subject vibrations while vehicle is running and thus tend to cause disconnection at connecting portion or to cause breakage. Such failure of wiring or control circuit causes drop out of the driver current for the proportioning valve assembly to cause unstability of the vehicular attitude. Particularly, for the type of the actively control system which is designed for controlling fluid pressure in respective working cylinder irrespective to the others, failure of wiring for one proportioning valve assembly may cause substantial variation of the vehicular attitude for making the vehicle substantially unstable condition.